1. Field of the Invention
The present invention relates to a laser irradiation apparatus and a laser irradiation method for crystallizing a semiconductor film using a laser light or for performing activation after ion implantation.
2. Description of the Related Art
In recent years, a technique of forming a TFT on a substrate has greatly progressed, and its application and development for active matrix semiconductor display devices have been advanced. In particular, since a TFT using a polycrystalline semiconductor film has higher field-effect mobility (also referred to as mobility) compared to a TFT using a conventional amorphous semiconductor film, it enables high-speed operation. Although the pixel is conventionally controlled by a driving circuit provided outside the substrate, it is therefore possible to control the pixel by the driving circuit formed on the same substrate where the pixel is formed.
Incidentally, as for the substrate used in the semiconductor device, a glass substrate is regarded as promising in comparison with a single crystal silicon substrate in terms of the cost. A glass substrate is inferior in heat resistance and is easily subjected to thermal deformation Therefore, in the case where a polysilicon TFT is formed on the glass substrate, in order to avoid thermal deformation of the glass substrate, the use of laser annealing for crystallization of the semiconductor film is extremely effective.
Characteristics of laser annealing are as follows: it can greatly reduce a processing time in comparison with an annealing method using radiation heating or conductive heating; and it hardly causes thermal damage to the substrate by selectively and locally heating a semiconductor or the semiconductor film, for example.
Note that the laser annealing method here indicates a technique of re-crystallizing the damaged layer formed on the semiconductor substrate or the semiconductor film, and a technique of crystallizing the semiconductor film formed on the substrate. Also, the laser annealing method here includes a technique applied to leveling or surface reforming of the semiconductor substrate or the semiconductor film. A laser oscillating apparatus applied thereto is a gas laser oscillating apparatus represented by an excimer laser, or a solid laser oscillating apparatus represented by a YAG laser. It is known that the apparatus performs crystallization by heating a surface layer of the semiconductor by irradiation of the laser light in an extremely short period of time of about several tens of nanoseconds to several tens of microseconds.
Lasers are roughly divided into two types: pulse oscillation and continuous wave, according to an oscillation method. In the pulse oscillating laser, an output energy is relatively high, so that mass productivity can be increased by setting the size of a beam spot to several cm2 or more. In particular, when the shape of the beam spot is processed using an optical system and made to be a linear shape of 10 cm or more in length, it is possible to efficiently perform irradiation of the laser light to the substrate and further enhance the mass productivity. Thus, for crystallization of the semiconductor film, the use of a pulse oscillating laser is becoming mainstream.
However, in recent years, in crystallization of the semiconductor film, it is found that grain size of the crystal formed in the semiconductor film is larger in the case where the continuous wave laser is used than the case where the pulse oscillating laser is used. When the crystal grain size in the semiconductor film becomes large, the mobility of the TFT formed using the semiconductor film becomes high. For this reason, a continuous wave laser has been attracting attention recently.
However, since the maximum output energy of the continuous wave laser is generally small in comparison with that of the pulse oscillating laser, desired power that is necessary for crystallization of the semiconductor film can be obtained by decreasing the size of the beam spot as about 10−3 mm2. However, the small beam spot needs long scanning times of laser beam per one substrate, thereby the substrate processing is ineffective.
If the beam spot is enlarged so as to improve the substrate processing efficiency, of course, energy density is decreased. In order to give the semiconductor film the total amount of heat necessary for crystallization, it is necessary to prolong the irradiation time. Consequently, the semiconductor film absorbs the laser light and conducts the heat to the substrate that cause thermal deformation of the substrate and TFT characteristics deteriorations due to diffusion of impurities from the glass substrate to the semiconductor film. In addition, when the substrate is heated, the crystallinity of the semiconductor film is hard to be even due to the heat of the substrate and the stability of TFT characteristics is disrupted.
In view of the foregoing, an object of the present invention is providing a laser crystallization method, and a laser irradiation apparatus using the a laser crystallization method, through which a substrate processing efficiency can be improved and damages to the substrate due to heat can be suppressed.